As others have said, general knowledge of encounters and your class is a very good start. Many group leaders on Oqueue or Openraid will ask for higher ilvl to increase success and lower the chance of failure. Experience is more often than not overlooked as it is very easy to say "10/14h" and to have never stepped foot into LFR. Some check armory profiles and most do not. Anyone who has the capacity to play their role well and understands their class can do Flex in my honest opinion.